The following invention relates generally to devices which measure the flow rate of fuel to an engine. More particularly, a flow loop is provided immediately preceding the fuel intake of an aircraft engine, the loop having first and second mass flow measuring devices which are averaged periodically and provide a visual display for a technician who is in the process of trimming fuel usage on the particular aircraft engine.
Mistrim of the total fuel flow in an F 15/F100 or F 16/F100 engine can cause engine augmentor blowouts, hard lights, stalls, and stagnations. With respect to overtrim, premature augmentor liner distress can also occur; in the event of undertrim augmented thrust is reduced.
To insure reliable augmentor operation, augmentor liner durability and performance, the engine manufacturer (Pratt & Whitney Aircraft) has defined a requirement to trim the F 100 engine total fuel flow to an accuracy of .+-.0.5%. Current systems provide an estimated accuracy of .+-.2.5% to 6.5%. Thus, a strong need exists to trim aircraft engines with the accuracy delineated hereinabove to preclude the possibility of mistrim as set forth above.
The practice of placing flow meters in pipe lines supplying liquid is quite common. However, the known techniques can be characterized in that they are either relatively inaccurate or when having the requisite accuracy, are too dependent on parameters not readily controlled in any place other than a laboratory setting. For example, whereas high accuracy volumetric flow meters are available, these engines consume fuel on a mass basis and therefore conversion to mass flow is necessary. The conversion process requires a density measure of the fluid and cognizance of the temperature dependence of the system, rendering a conversion procedure difficult unless performed with stringent controls not readily available in the field.